State of the art hearing instruments are usually either behind-the-ear (BTE) hearing devices, in-the-ear (ITE) hearing devices, in-the-canal (ITC) hearing devices or completely-in-the-canal (CIC) hearing devices. BTE hearing devices offer, due to the available space and the resulting possibility to use receivers of larger dimensions, and the opportunity to provide a rather high amplification and to obtain a usually satisfying sound quality. The sound transmission from the BTE device into the user's ear canal has to be done by a sound conduction tube which modifies the sound impression since the signal transmission characteristic is not homogeneous over the entire frequency range. Some of these modifications are wanted, others are unwanted and, if possible, are eliminated by means of acoustic filters in the sound conduction tube. ITE, ITC and CIC hearing devices, in contrast, have a shorter sound conduction tube or none at all. Also, ITC and especially CIC devices are barely visible from the outside and are therefore preferred by many users. However, they have the drawbacks of limited maximum amplification, limited battery lifetime and limited receiver quality, all due to the limited space available. Also the space in the ear canal has to be used efficiently and the ear canal essentially has to be closed by the device so as to minimise acoustic feedback due to the proximity of the sound outlet of the receiver and the sound inlet of the microphone. This plugging of the ear canal may cause undesirable effects, known as occlusion effect which has an impact on the perception of the wearer's own voice and on the wearing comfort.
In order to combine the advantages of BTE devices and of ITC and CIC devices, approaches have been proposed in which a BTE component is combined with an external component to be placed in the ear canal. The external component comprises the receiver.
In the following, different aspects of the mentioned application areas are considered.
Acoustical Performance
The quality of the acoustic signal transmission path of a hearing aid depends on four factors: The sensitivity of the acoustic-to-electric transducer (microphone), the performance of the signal processing unit, the response of the electric-to-acoustic transducer and the acoustic coupling between the electric-to-acoustic transducer output and the ear drum. Electric-to-acoustic transducers (“speakers”) in hearing instruments are often termed “receivers”, which term is used in the following for electric-to-acoustic transducers in or for hearing instruments of all kinds.
Microphones typically used in hearing aids have a sensitivity that is more or less flat within 10 dB in a frequency range between 100 Hz and 6 kHz. Variations from flat response occur both intentionally or undesired. At higher frequencies, there is often a rapid sensitivity deterioration, typically around 10 kHz, depending on the model. Typical receivers for hearing aids show frequency response curves with very characteristic structures due to the construction of the receiver (size, spout dimensions, etc.). Above 6 kHz typical receivers exhibit a significant fall off of the response curve.
For high quality sound perception, however, the ideal frequency response curve should mimic the natural acoustics of the ear in the range between 20 Hz and about 10 kHz, preferably even between 20 Hz and 16 kHz.
The German patent application publication DE 19634984 describes a hearing aid with several receivers integrated in the otoplastic (the component of the hearing aid which is specifically fit to the ear shape of the wearer and is worn in the ear canal or which at least protrudes into the ear canal). The multiple receivers are supposed to provide an improved sound quality. This hearing aid, however, has the drawback that a special new receiver technology has to be applied (multilayer foil technology) in order to fit the multiple receivers into the ear canal. This receiver technology has not proven to provide sufficient loudness and sound quality at all relevant frequencies and accordingly has not prevailed on the market.
Current hearing aids, therefore, still use only mainly one receiver for sound production. The acoustical performance is limited by the construction and size of the receiver, making it difficult to provide a high quality sound over a wide frequency range.
Connection Between a Hearing Aid Main Component and an External Receiver
Hearing devices, especially hearing aids, that comprise an external receiver placed outside the device's housing (most often in the ear canal or potentially the concha), have the problem that the connection link between the main component (most often the behind-the-ear component) and the receiver has to be adapted to the particular ear geometry of the wearer. Some manufacturers offer an external receiver component comprising a receiver embedded in a housing, a connection link made of flexible, partially pre-shaped and reinforced plastic tubing with two wires establishing the electrical connection between the receiver and the behind-the-ear (BTE) component. The mechanical and electrical interconnection is made by means of a two-pole plug-socket connector. Often, different lengths of a connection link are provided in a set. Such a set is for example described in WO 2004/025990 or in WO 2004/0010181. The disadvantage of such a set-up is that a plurality of interconnection links must be provided to fit the ear geometries. Hence the hearing professional must always have a set of connection links with different lengths in stock.
Other manufacturers offer behind-the-ear hearing aids with an external receiver, which, in contrast, come with a non-detachable interconnection and with one connection link length. The adjustment to the ear geometry causes the BTE component to change its position behind the ear accordingly.
EP 0158391 teaches a BTE with an external receiver with a connection link that is adjustable in length. The adjustment can be done either in the BTE component or in the receiver component. The basic principle of the invention is that the electric connection of the wires to the electronics in the BTE housing is fixed, i.e. non-detachable, while the connection link made of a tubular portion can be variably inserted into the hook or the housing of the BTE component, or into the housing of the receiver component.
The disadvantage of the solution proposed by EP 0158391 is that the external receiver assembly is not easily replaceable since the electrical conducting wires are attached to the electronics within the housing in a different way than the connection link. In addition, EP 0158391 does not reveal a method for securely attaching the connection link to the BTE component. In fact, an actual realisation of such a device showed severe problems with regard to the mechanical stability. Careless handling of the device caused a significant stress on the connection link, which may result in wire breaking.
DE 2721469 teaches a method for adjusting the length of the connection link by proposing a flexible print with conducting layers on both sides. This flexible print is folded and inserted in a (plastic) tube such to form an inner and an outer conducting layer. This tube can be cut to length by the hearing professional. The tube is then attached to the BTE component such that, for example, a spike on the BTE component makes electrical contact to the inner conducting layer and a fastening nut makes electrical contact to the outer conducting layer and at the same time provides the mechanical attachment. The mechanical reliability of such a solution is unknown. Also, a faulty length adjustment can not be reversed, since cutting to length is irreversible.
Fixation of a Hearing Instrument or a Component Thereof in the Ear Canal
Comfortable fixation of hearing aids that touch the highly sensitive skin in the portion of the ear canal beyond the isthmus (i.e. medial to the isthmus) has always been an issue with deeply fitted hearing instruments. This is particularly since for this mostly bony section of the ear canal physiological factors related to skin thickness and sensitivity are critical issues. Requirements for fitting such hearing instruments may comprise:
Enable the anchoring of a device safely and comfortably in the bony section, while minimising the pressure against the skin and avoiding friction during insertion, i.e. offer a solution which minimizes pressure and maximizes retention to prevent the walking out of the device (no working itself out)                Allow for atmospheric pressure equalization        Avoid an excessive humidity build-up in the occluded residual volume        Allow to place the device repeatedly at the same position        Avoid infections        
Patent literature contains several solutions for fixing a device deeply in the ear canal. The development of such solutions was mainly driven by completely-in-the-canal (CIC) related problems in minimizing acoustic feedback due to the dynamics of the ear canal and/or the wish for avoiding to make ear imprints. Some of the literature describes also acoustic seals that are intended specifically for sealing the bony portion of the ear canal.
Proposed fixation and sealing set-ups comprise:                Soft shells, made of foam or other elastic non porous material (such as silicone)        Soft shells filled with foam        Soft skin layer covering a rigid shell, foam sleeves, silicon rings, etc.        Tips (mushroom like, parachute like, multiple parachutes, ribs, seal rings, jelly or water filled flexible tips and air filled balloons or hearing devices with a mechanically expandable outer fixation structure)        
Such set-ups may be custom made or generic. Generic set-ups are solutions that do not require ear impressions.
Although literature related to deep canal (or peritympanic) devices describes devices that are solely anchored by the seal in the bony part, it is not known how reliable such designs are with regard to unwanted displacements or with regard to comfort.
U.S. Pat. No. 5,606,621 discloses a hybrid hearing device with a receiver component separated from the remaining parts of the hearing device, where the microphone, the battery and the signal processing unit is in a BTE-like assembly and where the receiver component (in a CIC like assembly) is placed in the ear canal such that it touches the bony portion of the ear canal. The CIC-like receiver assembly has a custom-made housing which makes contact to the ear canal walls.
US 2004/0047481, US 2004/0047482 and US 2004/0047483 disclose a fixation of the receiver component which is placed in the ear canal and which is connected through a cable with the remaining parts of a hearing device. This is done either with arms extending from the receiver component towards the ear canal walls, or alternatively with a foam disc, in which the receiver is placed, and which has a rim that touches the wall of the ear canal.
As to mounting outward of the isthmus, patent literature describes essentially two types of mounting schemes for hearing instrument components in the ear canal.                Universal-fit earpieces containing a hearing instrument component        Custom-shaped earpieces containing a hearing instrument component        
U.S. Pat. No. 5,002,151 discloses a universal fit earpiece with a user-disposable sleeve comprising soft polymeric retarded recovery foam that can be compressed to be freely insertable into a person's ear and allowed to recover to become wedged in the canal. The sleeve is detachably attached to the ear piece of a hearing aid, which includes any sound transmission device. Preferably, the sleeve is detachably attached to the ear piece by mating of screw threads on the sleeve and the ear piece. The ear piece may be a separate component from the hearing aid. The component is made either of rigid or flexible plastic and has connecting portions of various lengths depending on the depth of insertion of the sleeve into the canal. The sleeve may be of various lengths depending on the depth of insertion into the ear canal desired. The sleeve/ear piece assembly may also have a layer of sound transmitting scrim over its central opening to minimise penetration of the connecting portion past the end of the sleeve. A venting system is proposed by means of at least one flute on the exterior surface of the foam sleeve. Deformation of the tip due to the ear canal dimensions will cause the vent cross sections to change unpredictably, such that the acoustic coupling changes correspondingly.
U.S. Pat. No. 5,887,070 discloses an insert earphone in which a piece of foam material is used to resiliently mount a receiver within a chamber portion of a one-piece plastic housing member. The receiver has an output port extending through a central aperture of the piece of foam material and into one end of a passage defined by a tubular portion of the housing member with a damper being disposed in the other end of the passage. The tubular portion is inserted into an ear tip or other coupling device and has an enlarged diameter end section to achieve a locking action. The unitary housing is attached to an ear tip which also made of foam to adapt to the ear canal geometry. No venting is provided for.
U.S. Pat. No. 6,129,174 discloses an acoustic coupler adapted for use with an intra-canal receiver module which can be deeply inserted into the ear canal of the user while making minimal contact with the walls of the ear canal. The minimal contact feature of the invention allows the acoustic coupler to seal the ear canal acoustically and anchor a hearing device at an optimal depth within the ear canal, while maximizing the user's comfort. The acoustic coupler is manufactured from a soft, pliable elastomer that allows it to conform readily to the shape of the ear canal. The acoustic coupler incorporates structural supports that allow the coupler to maintain an acoustical seal and withstand the inward pressure of the ear canal wall while making minimal contact with the ear canal. A vent pathway for control of occlusion effects is also provided.
WO 99/07182 discloses several tips which are attached to a universal receiver housing. None of them is intended to be custom-shaped. The material properties of the acoustic coupler are described as soft and compliant to adapt to the variable and irregular shape of the human ear canal.
WO 01/69972 discloses a flexible tip for a hearing aid including a mushroom shaped tip, an inner portion having a bore and a receiver mounted within the bore. The receiver can be housed and sealed within a receiver housing. The receiver housing can include a spring having a high compliance along a longitudinal axis and transverse axis, to provide flexibility in the flexible tip. The spring can also have a high stiffness along a radial direction about the circumference of the spring to provide support of the flexible tip from radially directed loads.
U.S. Pat. No. 5,572,594 discloses an ear canal device holder for devices other than speaker/microphone amplification systems that are to be inserted into the canal of the human ear. The device holder is made of a flexible silicone material comprising a body and structural support element(s) such that the device is held within the body of the holder and the body and device are secured in the ear by the structural element(s). In addition the device holder minimises the attenuation of sound waves that pass through the ear canal to the tympanic membrane, while maximising comfort and secure fit.
US 2004/0047483 discloses a unitary receiver housing from that is suspended in the ear canal by means of radially extending structural elements such as arms or discs. With the use of arms, which are bent inside the ear canal in order to accommodate the individual shape of the ear canal, open fitting is therefore optimally supported in terms of effective vent size.
WO 2004/100608 and US 2004/0215053 disclose balloon expandable and self expandable hearing devices and receiver modules, being further examples of universal-fit set-ups.
The disadvantage of all universal-fit s receiver or hearing device holder solutions is that the positioning of the receiver within the ear canal is not reproducible. In some cases, such as is described in US 2004/0047483, the effective vent size is a priori not known since it is defined by the ear canal geometry which is intentionally not being directly or indirectly measured by taking ear impressions. The resulting uncertainty of positioning and venting causes variations of the acoustic coupling, which limits the acoustic performance of the hearing aid: the hearing aid's gain may not be optimised for the particular geometry the earpiece assumes when inserted in the ear canal. Further, the assembly is usually not firmly positioned in the ear canal and may walk out and/or may cause a tickling sensation, either during jaw movements or when the assembly is touched from the outside with fingers.
CH 664057 discloses hearing aids with custom shaped components. The subject is a BTE hearing aid with transducers in separate, sound proof housings made such that sound can reach the microphone or be radiated by the receiver only through dedicated openings, i.e. the microphone inlet or the receiver outlet. In one embodiment, the receiver housing is placed within the otoplastic. The otoplastic may have a (conventional) venting.
However, such a set-up with a custom shaped CIC component has the disadvantage that the volume between the CIC component and the tympanic membrane is largely closed which reduces the wearing comfort and increases the occlusion effect. Also, the receiver has to be assembled in special, custom-made set-up. It is not possible to exchange receivers except by replacing the entire CIC component.
US 2004/025990 describes an earpiece auditory device including a behind-the-ear (BTE) component, which includes processing circuitry. In an embodiment, the device also includes a completely-in-canal (CIC) component, shaped to fit into the ear canal of the user such that it touches the bony portion of the ear canal. In some embodiments, the CIC component includes either a universal fit or a custom fit ear mold. The custom fit ear mold can be fabricated using a rapid prototyping technology, in which the contours of the user's ear canal are scanned, and the scan data is used either directly or indirectly to replicate the ear canal contours of that user into the custom fit ear mold. In some embodiments, the ear mold is detachably interconnected with a speaker module, preferably using either an intermediate sleeve or a detachable locking pin assembly. In another embodiment, the speaker module is permanently encapsulated within the ear mold. It is mentioned, that in one embodiment the CIC unit has an open mold configuration or a vent, meaning the ear canal of a user is at least partially open when CIC unit is inserted so deep into user's ear canal as to touch the bony portion.
However, configurations suffer from the drawback that even if the configuration is an open mold configuration or has a vent, it is may be difficult to provide enough air exchange between an inner portion of the ear canal and the outside. Also, such state-of-the-art custom shaped earpieces may usually not be made compressible and cause discomfort during jaw movements.